Democrat Russ Feingold has the backing of his party's standard-bearers, President Barack Obama and Vice President Joe Biden.
Feingold, D-Middleton, announced the endorsements Thursday. The former three-term senator is facing first-term Republican Sen. Ron Johnson of Oshkosh in Wisconsin's U.S. Senate race.
"Russ is an honest, independent-minded public servant who's not driven by what's politically expedient; he's driven by what he knows is right for the people of Wisconsin," Obama said in the statement.
Feingold, Obama and Biden go back together, having all served in the Senate from 2005 to 2009.
Feingold also was an early supporter of Obama in his 2008 presidential bid.
Biden alluded to their shared history in his statement, saying "we've fought together to grow our economy, raise wages for American workers, protect Social Security and clean up our campaign finance system."
Feingold returned the favor, saying Obama and Biden have "guided us to a stronger economy" since they took office in 2009.
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Republicans sought to cast Obama's support as a a negative for Feingold.
Johnson's campaign planned to use the Obama endorsement to solicit contributions from supporters, saying it shows "we're up against the full force of Washington liberals in this fight."
"Wisconsinites deserve a leader to repair President Obama's failures -- not a proxy to rubber stamp them," said Andrea Bozek, spokeswoman for the National Republican Senatorial Committee.